Low-noise RF preamplifier (especially for nuclear magnetic resonance system)

Abstract

A low-noise radio-frequency preamplifier, especially suited for use in the reception channel of a nuclear magnetic resonance spectrometer and the like, utilizes a low-noise input stage having a plurality of bipolar junction transistors effectively connected in radio-frequency parallel, but with each of the plurality of parallel bipolar transistors isolated from one another for direct current flow through at least one electrode thereof.

What we claim is: 
     
       1. A radio-frequency (RF) preamplifier comprising: a linear low-noise input stage having a plurality N of bipolar junction transistors having like electrodes effectively connected in RF parallel, and with each of the base electrodes of said plurality of parallel bipolar junction transistors being isolated from one another for direct current flow.   
     
     
       2. The preamplifier of claim 1, further comprising at least one additional gain stage serially connected after the low-noise input stage. 
     
     
       3. The preamplifier of claim 2, further comprising means for switch selecting the total preamplifier gain. 
     
     
       4. The preamplifier of claim 3, further comprising means for independently setting the collector current in each of the paralleled transistors. 
     
     
       5. The preamplifier of claim 2, further comprising means for independently setting the collector current in each of the paralleled transistors. 
     
     
       6. The preamplifier of claim 1, further comprising means for independently setting the collector current in each of the parallel transistors. 
     
     
       7. The preamplifier of claim 1, wherein the input stage transistors are each biased to have an optimum input impedance of about NxZ 0 , where Z 0  is a preselected characteristic impedance. 
     
     
       8. The preamplifier of claim 7, wherein N=2, Z 0  is about 50 ohms and the optimum source impedance of the preamplifier is between about 25 ohms and about 70 ohms at a predetermined frequency. 
     
     
       9. In combination in a nuclear magnetic resonance (NMR) sample investigation system: an antenna for receiving an NMR response signal; and a radio-frequency (RF) preamplifier comprising a linear low-noise input stage having a plurality N of bipolar junction transistors having like electrodes effectively connected in RF parallel and directly connected to said antenna without any additional impedance-matching network therebetween and with each of the base electrodes of said plurality of parallel bipolar junction transistors being isolated from one another for direct current flow. 
     
     
       10. The combination of claim 9, wherein said preamplifier further comprises at least one additional gain stage serially connected after the low-noise input stage. 
     
     
       11. The combination of claim 10, further comprising means for switch selecting the total preamplifier gain. 
     
     
       12. The combination of claim 11, further comprising means for independently setting the collector current in each of the paralleled transistors. 
     
     
       13. The combination of claim 10, further comprising means for independently setting the collector current in each of the paralleled transistors. 
     
     
       14. The combination of claim 9, further comprising means for independently setting the collector current in each of the paralleled transistors. 
     
     
       15. The combination of claim 9, wherein the input stage transistors are each biased to have an optimum input impedance of about NxZ 0 , where Z 0  is a preselected characteristic impedance. 
     
     
       16. The combination of claim 15, wherein N=2, Z 0  is about 50 ohms and the optimum source impedance of the preamplifier is between about 25 ohms and about 70 ohms at a predetermined frequency.